Writing comics is a specific skill. Not every prose writer or screenwriter can do it. Writing prose is a specific skill. Not every screenwriter or comics writer can do it. Screenwriting is a specific skill not every prose or comics writer can do it.

Each has different goals and each requires a different understanding of how to execute a story. One primary difference is novels are complete unto themselves. Scripts are only a part of the eventual whole- blueprints for the house.

A lot can be learned by doing, true, but I've known a stack of screenwriters who can't crack prose and a few comics writers who can't seem to cross into screenwriting. Prose writers, too, can have trouble with the rigid Spartan structure of a screenplay or a comic book script.

There's a reason some writers gravitate to different media to express their tales. Some fit better than others.